fix(Compiler): add an error when a directive is null or undefined

fixes #1908

modules/angular2/test/reflection/reflector_spec.js

@@ -121,6 +121,11 @@ export function main() {
         reflector.registerType(TestObj, {"annotations" : [1,2]});
         expect(reflector.annotations(TestObj)).toEqual([1,2]);
       });
+
+      it("should work for a clas without annotations", () => {
+        var p = reflector.annotations(ClassWithoutAnnotations);
+        expect(p).toEqual([]);
+      });
     });
 
     describe("getter", () => {
@@ -165,4 +170,4 @@ export function main() {
       });
     });
   });
-}
+}